This invention relates to color image processing. More particularly, this invention relates to methods and apparatus for editing color profiles, such as color profiles for color output devices.
A color profile generally includes a transform from a profile connection space, such as Commission Internationale de l'Eclairage (“CIE”) XYZ color space, CIE L*a*b* color space, or other similar color space, to a device space (the “reverse transform”), and a transform from the device space to the profile connection space (the “forward transform”). For example, a color profile for an output device (referred to herein as an “output profile”), includes transformations between the color space of the output device and the profile connection space. Thus, if an output device is a four-color printer that uses cyan, yellow, magenta and black (“CMYK”) colorants, the reverse transform may be used to convert device-independent L*a*b* color values to equivalent device-dependent CMYK colorant values, and the forward transform may be used to convert device-dependent CMYK colorant values to device-independent L*a*b* color values.
To create an color profile for an output device, such as a color printer, copier, offset press, display device, or other similar output device, a test chart including numerous color patches is printed by or displayed on the output device. Exemplary test charts include IT8, ECI2002, and other similar test charts. Next, a measurement device, such as a calorimeter, spectrophotometer, spectrocolorimeter, or other similar device is used to obtain spectral or colorimetric data that describes each printed or displayed patch. The measured data are then provided to profile generation software, which calculates a color profile based on the spectral or calorimetric measurements. Profile generation software may be any conventional profile generation software, such as the EFI Color Profiler software, licensed by Electronics for Imaging, Inc., Foster City, Calif., U.S.A., or other similar profile generation software.
The created color profile may then be used to print or display color images on the profiled output device. A user may find, however, that printed or displayed images using the color profile produce unacceptable results. For example, a printed image may have color shifts, such as white to blue shifts toward purple. Alternatively, the user may perceive hue shifts along blends, or may notice that the gray balance is not quite right. In the past, a user facing such problems might attempt to manually edit the color profile. Often times, however, an inexperienced user would not know how to edit the color profile, or might edit the profile in a way that did not solve the problem, made the problem worse, or created new problems.
To overcome these problems, some previously known software programs have been developed to simplify the color profile editing process. The programs typically permit a user to select a color profile for editing, and a reference image for evaluating the effects of the edits. Some previously known software programs display “before” and “after” soft proof images to allow a user to visually see the effects of the edits on the reference image. The applications also typically provide tools for editing the profile. For example, some previously known profile editing programs include “curves” tools or slider bars that allow a user to create global shifts in the lightness, saturation and colors in the image. In addition, some previously known profile editor tools allow a user to change specific color regions in the color profile. Such “selective color edit” tools typically provide a sampling tool to allow a user to sample a color from the reference image, and also provide slider bars that allow a user to change individual color values of the sampled color, such as the sample's L*a*b* values.
Although previously known color profile editor programs have many useful features, the applications often have several shortcomings. In particular, previously known color profile editing tools provide limited information about the profile being edited, and the available gamut of the associated output device. For example, although several programs display “before” and “after” soft proofs of the reference image using the unedited and edited profiles, the programs do not provide soft proofs of the original source image. Further, previously known solutions do not display color difference information between colors in the reference image and the printed or displayed image using the edited profile, or between colors in the printed or displayed image using the unedited and edited profiles.
Moreover, previously known selective color edit tools often restrict the manner in which information may be displayed regarding the color profile, and the manner in which the color profile may be edited. For example, some selective color edit tools allow a user to make selective color edits using L*a*b* slider bars, but provide limited information about the available colors in the color gamut of the output device. For example, a user may desire to increase the a* value of the selected color, but the desired value may not be available at the existing L* and b* values. Although the program may display a cross section of the profile for the given L* value, the program may not display any other information about the output color gamut that would allow the user to select available edited color values.
In addition, although each user may have an individual preference for perceiving color, previously known color profile editing systems typically limit the presentation of color information to the user. For example, one user may be more comfortable visualizing color in terms of hue, lightness and chroma, whereas another user may prefer to work with color in XYZ values. Nevertheless, some previously known color profile editors only display color as L*a*b* values, and thus force all users to work with color values in a single coordinate system. For a user who is not comfortable working in L*a*b* values, the color profile editing program may be less useful and effective.